export default class LetterList extends React.Component {
  render() {
    let lists = [
        {
          key: "One",
          data: [{letter: 'a'}, {letter: 'b'}]
        },
        {
          key: "Two",
          data: [{letter: 'c'}, {letter: 'd'}, {letter:'e'}]
        }
        {
          key: "Three",
          data: [{letter: 'f'}]
        }
      ]

    return (
      <View key="letterList" >
        <SectionList
          renderItem={({section}) =>
            (
              <FlatList
                data={section.data}
                renderItem={({item}) => <Text>{item.letter}</Text>}
              />
            )
          }
          renderSectionHeader={({section}) => <Text>{section.key}</Text>}
          sections={lists}
        />
      </View>
    );
  }
}

One
ab
ab
Two
cd
e
cd
e
cd
e
Three
f